Ion implanters are used to modify semiconductor substrate material by implanting ions of a desired species into the substrate, thereby altering the properties of the substrate.
Ion implantation takes place in a vacuum chamber, in which a substrate or wafer, into which ions are to be implanted, is mounted on a holder. Ions are usually directed from an ion source towards the substrate in the form of a beam, and the beam cross-sectional area is usually smaller than the wafer to be implanted. In order to implant evenly over the entire surface of the substrate wafer, some relative scanning between the substrate and the beam is provided.
Various scanning arrangements have been proposed and used in the prior art, including two-dimensional beam scanning with the substrate wafer held still on the holder, hybrid scanning in which the beam is scanned in a first direction whilst the wafer is mechanically passed through the scanned beam in a second direction, and mechanical scanning in which the beam is maintained substantially steady and the substrate is mechanically scanned in two dimensions. Examples of mechanical and hybrid scanning arrangements include U.S. Pat. No. 5,641,969, U.S. Pat. No. 5,898,179, U.S. Pat. No. 6,555,825, WO 03/088303 and WO 2004/001789.
It is a usual objective in ion implantation to ensure that a desired dosage of the desired species is delivered uniformly over the surface of the or each substrate wafer being implanted. The total dose per unit area being delivered to the substrate must also usually be carefully controlled. To this end, the current of ions in the ion beam is measured and the scanning arrangement is adjusted to ensure each part of the wafer is exposed to the beam for an appropriate period of time so as to receive the desired total dose.
Beam current is usually measured by means of a faraday collector. This device ensures that secondary electrons which may be generated when beam ions impinge on a collection surface, are not allowed to escape the measuring device, so that the charge collected by the faraday collector is a correct measurement of the total number of ions collected.
Unfortunately, the current of desired ions delivered to the wafer in the beam of a typical implanter is not completely constant. The beam current may experience relatively slow drift, and additionally may experience sudden and short term changes. These short term changes, typically referred to as glitches, may be the result for example of an arcing event at some location along the beam line. Whereas the slow drift of ion beam current may result in a change in current of a few percentage points over many minutes, glitches typically have a fast onset, a duration up to a few hundred milliseconds, and a fast recovery. Ideally, it would be desirable to monitor accurately the current of ions in the beam throughout the implant process. However, it has proved impracticable to make a continuous and reliable current reading of beam ions impinging on the substrate during the implant process, so that in practice only those ions which bypass the substrate during the scanning procedure can be effectively monitored with a faraday collector located behind the substrate holder.
However, various techniques have been proposed for obtaining ion beam current readings at intervals during an implant process. For example, U.S. Pat. No. 4,234,797 discloses a batch implanter using a scan wheel having a number of substrates mounted around the periphery of the wheel. The substrates are mechanically scanned through the beam by rotation of the wheel and simultaneously translating the wheel axis. A slot is provided in the wheel between a pair of adjacent substrate supports to allow beam ions to pass through the wheel as the slot passes over the beam during rotation of the wheel. In this way, a measure of beam current can be obtained once each wheel rotation. U.S. Pat. No. 6,646,276 discloses a scan wheel formed of multiple substrate supports carried on spokes so that there are spaces between each adjacent pair of supports. This patent also discloses taking ion beam measurements throughout the implant process as the wheel is scanned through the beam. The beam current is sampled at a sufficiently fast rate to obtain values of the current of beam ions passing between adjacent wafer supports and spokes of the scan wheel.
An additional problem with obtaining an accurate measure of beam current during the ion implantation process is that the faraday collector used to monitor beam current can only detect and respond to charged particles in the beam. There is a tendency for beam ions to become neutralised primarily by charge exchange events with residual gas atoms in the region through which the beam passes before impinging upon the substrate to be implanted. The presence of neutralised atoms or molecules of the desired species in the beam impinging upon the substrate is not a problem, provided the beam ions became neutralised only after they had achieved the final desired energy and beam direction towards the substrate to be implanted. However, the presence of neutralised particles in the ion beam does present a problem for obtaining accurate measurements of beam current, since neutral particles will not be measured.
It has been understood for many years that the proportion of neutral particles in the ion beam is related to the pressure of residual gas through which the ion beam passes before impinging on the substrate. A steady state background residual gas pressure in the evacuated chamber in which ion implantation is conducted can be tolerated, since the resulting proportion of neutral particles in the ion beam is likely to remain substantially constant and can be corrected for empirically, e.g. by a scalar multiplier. However, the implantation process on a substrate wafer causes material, typically photoresist used for masking the wafer, to outgas from the wafer surface. This results in an unpredictable increase in the residual gas pressure in the process chamber, with consequent increase in the proportion of neutral particles in the ion beam. As a result, ordinary measurements of ionic current received by a faraday, taken during the time when the substrate is being scanned relative to the ion beam, will tend to under measure the beam current by an unpredictable amount. As a result, the total dose of desired species implanted in the substrate by the process becomes unpredictable.
Various proposals have been made to counteract the effect on beam current measurement of variations in the residual gas pressure caused by outgassing during implantation. For example, U.S. Pat. No. 6,297,510 discloses monitoring the residual gas pressure in the process chamber of an ion implanter whilst making multiple beam current measurements between process scans, in order to apply a computed correction to the measured beam current. Arrangements for correcting measured beam current during scanning, which involve obtaining residual gas pressure measurements, are also disclosed in U.S. Pat. No. 4,539,217, U.S. Pat. No. 4,587,433 and U.S. Pat. No. 5,760,409.
The increase in residual gas pressure resulting from outgassing during the implant process is reduced again by the vacuum pumping system quite quickly once outgassing ceases, e.g. once the substrate is no longer being scanned through the ion beam. Accordingly, ion current measurements taken before and after a scanning process can be reliably accurate, so long as sufficient time is allowed for residual gas pressure to be pumped back down to the nominal value. Accordingly, it has been established practice with batch implanters using a scan wheel to make accurate beam current measurements during an implant process each time the scan wheel axis is at one extreme of its reciprocating motion at which the substrate holders and substrates thereon are spinning completely clear of the ion beam. So long as the scan is paused in this position for a sufficient time for the vacuum system to reduce the residual gas pressure back to nominal, an accurate beam current reading can then be taken. This procedure allows a fresh and accurate beam current measurement to be taken at intervals of a few seconds through an implant process. The implant process may comprise multiple reciprocations of the wheel axis passing the substrates mounted on the wheel periphery repeatedly through the ion beam, while the scan wheel is rotated at a fast rate, typically 200–1250 rpm. Thus, rotation of the scan wheel causes the beam to sweep repeatedly over each substrate at a fast rate (approximately every 50 millisecond), while these fast sweeps over the substrate pass radially over the substrate with reciprocation of the wheel axis at a slower repetition rate (typically a few seconds). These repeated beam current measurements during an implant process can be sufficient to allow the process to be controlled to compensate for slow changes in the beam current resulting from drift of ion source parameters for example. However, these accurate beam current readings taken at the end of each slow scan do not normally detect short term glitches of the beam.
U.S. Pat. No. 6,646,276 proposes obtaining samples of the beam current passing the substrate holders of a spoked wheel type batch implanter at a sufficiently fast sample rate (1 kHz or higher) so as to achieve multiple current samples of beam ions passing the substrate holders of the scan wheel between adjacent pairs of substrate holders. The patent proposes comparing beam current values obtained between each pair of substrate holders with preceding current values, in order to determine a sudden reduction of beam current, likely to be indicative of a beam glitch. A record is then maintained of the number of beam glitches affecting each substrate of the batch being implanted, in order to provide an indication of substrate wafers in the batch likely to be defective or inadequately implanted. However, the procedure disclosed in this patent provides only an indication of the number of defective fast sweeps of the beam across each substrate, thereby providing only a rough and ready indication of the likelihood of a defective or inadequately implanted substrate.